Very interesting. Of course this is purely Conservative and Reform.
I'm sure that die hard Ultra Orthodox wouldn't even give this a blind eye.
The diets of the Ashkenazi and Sephardic are different because of their respective environments.
For instance the Mediterranean Jews can eat beans on Passover while the Eastern European Jews do not.
The five grains: wheat, barley, oats, rye and spelt, mentioned are forbidden for only one reason, and that being that if they will come into contact with water, ferment and create a gas which will make whatever is made from the grain, rise.
The ancient tradition of Passover is that when the Israeli people fled Egypt, they had no time to allow their bread to rise, so they rolled the dough into flat cakes and baked them in the hot sun, hence the first "Matzo."
The tradition continues during the observance of the holiday when no food that can possibly ferment and rise is eaten for 7 days.